1. Field of the Invention
The present invention relates to a method for making a polycrystalline thin film having a highly oriented grain structure and a method for making oxide superconductor on such a thin film base and an apparatus for making said polycrystalline thin film.
2. Description of the Related Art
Oxide superconducting materials discovered in recent years are excellent superconductors having a critical temperature higher than the liquid nitrogen temperature, but many problems remain to be resolved before such oxide superconducting materials can be used as practical superconductor devices. One such problem is that the critical current density is low for these oxide superconductors.
The problem of low critical current density is caused largely by the anisotropic electrical properties of the crystals in the superconductor themselves, and especially, it is known that an electrical current can flow relatively easily in the directions of a- and b-axes but has a difficulty flowing in the c-axis direction. Therefore, to deposit an oxide superconductor on a substrate base and to use such a material as a superconducting device, it is necessary to form an oxide superconducting layer on a substrate base whose grain structure has a highly-developed controlled orientation, and furthermore, the a- and b-axes must be made to align in the direction of the current flow while the c-axis is oriented in another direction which does not impede the current flow.
Various methods have been tried in the past to deposit oxide superconductors of a suitable orientation on a substrate such as plate and metal tape. One such method currently in use is a thin film growth technique based on sputtering of superconducting oxide material on a single crystal substrate base of a material such as MgO or SrTiO3 which have similar crystal structures to oxide superconductors.
Using such a single crystal substrate material to grow a thin film by sputtering, for example, it is possible to deposit a single crystal layer having an excellent directionality in the crystal orientation, and it is known that a high critical current density exceeding several hundred thousand of amperes per square centimeter can be achieved in the superconducting oxide layer formed on the single crystal base.
To use the oxide superconductor as an electrical conductor, it is necessary to deposit a uniformly oriented superconducting layer on a longitudinally extending base, for example a tape base. However, when such a layer is deposited on a metal tape, because the substrate metal itself is a polycrystalline material and its crystal structure is quite different from that of the oxide material, it is virtually impossible to produce a highly oriented superconducting layer. Additionally, because of heat treatments necessary to develop superconducting properties, diffusional reactions which can occur between the superconducting oxide layer and the metal tape base disturb the interface structure and degrade the superconducting properties.
For these reasons, general practice is to form a superconducting layer on top of a sputtered intermediate layer, comprised by materials, such as MgO or SrTiO3, on a metal tape. However, the problem with a superconducting oxide layer formed on such an intermediate layer is that it exhibits only a low critical current density (for example, several thousand to tens of thousand A/cm2). This problem is thought to be due to the following causes.
FIG. 15 shows a cross sectional view of a sputtered superconducting oxide layer 3 formed on top of an intermediate layer 2 on a base 1 of a metal tape, for example. The superconducting oxide layer 3 is a polycrystalline layer and is comprised by numerous randomly oriented grains 4. Close examination of the individual grains 4 reveals that although the c-axis of each grain 4 is at right angles to the base, both a- and b-axes are oriented in random directions.
When the a- and b-axes are randomly oriented in the neighboring grains, quantum coupling in the superconducting state is destroyed at the grain boundaries which are irregular lattice structures, and the result is that the superconducting properties, especially the critical current density become seriously affected.
Also, because the underlying intermediate layer 2 is polycrystalline without the uniform orientation of a- and b-axes, the superconducting oxide layer 3 becomes a polycrystalline layer of randomly oriented a- and b-axes, and the growth of the layer 3 occurs in conformity with the underlying random orientation nature of the intermediate layer 2.
Technology of growing an oriented film of various materials on polycrystalline substrate is utilized in fields other than the above-mentioned oxide superconductor field. For example, they are useful in optical thin films, opto-magnetic discs, circuit bords, high frequency waveguides and signal filters, as well as in cavity resonators, but in every field, an important requirement is to produce a polycrystalline film having a highly developed crystal orientation of a uniform quality. In other words the quality of the thin film for optical, magnetic and circuit applications would be expected to be better if the film can be formed on a polycrystalline base having a controlled grain orientation, and it would be even more desirable if a properly oriented films for such applications can be deposited directly on the substrate base.
For these reasons, the present inventors have been investigating processes of forming a polycrystalline layer of yttrium-stabilized zirconia (abbreviated to YSZ hereinbelow) on a metal tape and subsequently depositing superconducting oxide layer on the polycrystalline layer to produce an oxide superconductor of superior properties.
These efforts have resulted in publications of the following patent applications, for producing a polycrystalline film of a controlled orientation and oxide superconductors formed thereon: JPA, First Publication, H4-329865 (Application No. H3-126836); JPA, First Publication, H4-331795 (Application No. H3-126837); and JPA, First Publication, H6-145977 (Application No. H4-293464).
These studies have shown that irradiating ion beams at an inclined angle to the YSZ layer being formed enabled to obtain a superior orientation control of the grains.
Concurrent with these investigations, studies have been conducted on how to produce polycrystalline thin films and oxide superconductors on an extending or large area substrate. As a result of the accumulated efforts, not only a method of making. polycrystalline thin films to provide a superior control over the crystal orientation but also a method of forming an oxide superconductor of superior superconducting properties on top of such a substrate base have been developed.
It is an object of the present invention to continue to enhance the work carried out to date by providing a method for making a highly oriented polycrystalline substrate base and then to form an oxide superconductor of controlled crystal orientation on the substrate base so that not only the c-axes of the polycrystals are orientated at right angles to the film surface but the a- and b-axes are also well-aligned in a horizontal direction parallel to the film surface, thereby leading to an oxide superconductor having a superior critical current density and improved superconducting properties. Another object is to present a deposition apparatus to be used with the method.
The object has been achieved in a method for making a polycrystalline thin film by depositing particles emitted from a target on a substrate base so as to form a polycrystalline thin film comprised by elements constituting the target while concurrently irradiating the particles being deposited on the substrate base with an ion beam generated by an ion source, at an angle of incidence, in a range of 50 to 60 degrees to a normal to a film surface, and maintaining a film temperature at not more than 300 degrees Celsius.
In the method presented above, the target may be comprised by yttrium-stabilized zirconia.
In the method presented above, it is preferable that the polycrystalline thin film has a film thickness of not less than 200 nanometers.
The object has been achieved also in a method for making an oxide superconducting body by depositing particles emitted from a target (36) on a substrate base (A) so as to form a polycrystalline thin film (B) comprised by elements constituting said target (36) while concurrently irradiating said particles being deposited on said substrate base with an ion beam generated by an ion source (39), at an angle of incidence in a range of 50 to 60 degrees to a normal (H) to a film surface, and maintaining a film temperature at not more than 300 degrees Celsius to produce a film material to be used by itself or as a substrate base for a functional thin film, and then depositing a superconducting layer on top of the polycrystalline thin film.
In the above method, the target may be comprised by yttrium-stabilized zirconia.
In the above method, it is preferable that the polycrystalline thin film has a film thickness of not less than 200 nanometers.
According to the present method, a crystal aligning ion beam is irradiated at an angle of incidence of between 50 to 60 degrees to a normal to the film surface onto the particles which are emitted by the target and being deposited on the substrate base, maintained at a deposit temperature of not more than 300xc2x0 C. This process enables to produce a YSZ thin film comprised by polycrystalline grains whose c-axes are oriented at right angles to the film surface and a-axes (or b-axes) are oriented within a planar alignment angle of less than 35 degrees of a-axes (or b-axes) in the neighboring grains.
It is believed that this is a result of the action of the ions in the crystal aligning beam in removing unstable atoms which are oriented in non-aligned directions so that only those stable atoms which are oriented in the specified direction tend to remain on the substrate base. The result is a production of polycrystalline thin film of superior grain alignment. By controlling the deposition temperature at 300xc2x0 C. or lower, effects of atomic mobility and lattice vibrations are reduced relative to the bombarding effects of the ion beams, so that a polycrystalline thin film of superior orientation control can be produced.
Therefore, by using such a substrate base for growing other functional thin films, devices having superior functional properties can be produced. In other words, if the functional film is a magnetic film, a magnetic thin film device of superior performance can be produced. If the functional film is an optical film, then an optical device of superior performance can be produced.
An example of a material which can be emitted by the target is yttrium-stabilized zirconia, and the product will be a highly oriented YSZ polycrystalline thin film.
Further, by limiting the thickness of the produced film to be not less than 200 nm, the grown thin film will have a sufficiently uniform orientation.
By forming a superconducting layer on the polycrystalline substrate base prepared according to the present method, a superconductor having an excellent consistency in grain orientation can be produced, thereby providing superior critical current density and superconducting properties.
An example of the polycrystalline substrate base which can be used is yttrium-stabilized zirconia. Also, by limiting the thickness of the polycrystalline base to be not less than 200 nm, a superconductor having superior performance properties will be formed on the substrate base of highly oriented polycrystal grains.
The present method is achieved by using an apparatus comprising: a deposition chamber for depositing a polycrystalline thin film on a substrate base and for housing following component devices; a feed spool for feeding a tape base in a longitudinal direction; a take-up spool for winding the tape base forwarded from the feed spool; a base holder disposed between the feed spool and the take-up spool for guiding the tape base while being in contact with a back surface of the tape base; a target disposed opposite to a front surface of the tape base being guided in the base holder for depositing particles emitted from the target; an ion source disposed opposite to the front surface for radiating an ion beam towards the front surface at an angle of incidence selected from a given range of angles; and a cooling device for cooling the substrate base through the base holder.
The apparatus is arranged so that it is possible to irradiate the particles being deposited on the substrate base in a tape form at an optimum angle, while the tape is being driven by a feed spool and conveyed over a base holder to be wound on the take-up spool. The depositing film can be kept at a suitable low temperature by cooling the base holder to promote the grain-aligning-effect of the ion beam irradiation, so that a polycrystalline thin film having superior crystal orientation can be produced.
In the apparatus, the cooling device is comprised by a hollow pedestal for attaching the base holder; and a cooling pipe attached to the pedestal and communicating an interior space of the pedestal with an exterior space by entering through an external wall of the deposition chamber.
Accordingly, because the cooling device can be operated independently of the low-pressure deposition chamber, the growing thin film can be cooled through the base holder so that the grain-aligning-effect of the ion beam irradiation can be applied more effectively to the depositing particles to produce polycrystalline thin film of superior orientation control, while controlling thermal vibrations and other adverse effects which disturb crystal orientation.
Furthermore, effective cooling of the growing thin film is obtained by providing inflow and outflow pipes to prevent stagnation of spent cooling liquid or gas.
In the apparatus, the cooling pipe has a double wall structure comprised by an inlet pipe for admitting a coolant by communicating with the interior space, and an outlet pipe surrounding the inlet pipe to communicate the interior space with the exterior space.
By providing a double-wall structure for the cooling device, the gas or liquid in the inflow pipe can be cooled by the liquid or gas being expelled from the base holder, thereby preventing a temperature rise in the inflow pipe so that effective cooling of the growing thin film can be maintained throughout the deposition process.